It's Mann vs Machine, named after Mann Co. Mann up gives you a small chance of getting cosmetic hats and misc items after every mission you complete. Completing the entire set of missions once will give one random botkiller weapon, which also tracks your number of kills on other players.
Alternatively, if you rely on your parents to sustain you economically, you could go on a regular MvM server and vote to change the difficulty to advanced. You won't get anything but it will have the same difficulty as the Mann up missions, according to what other people said.